I have to book some flights for my family and Air Asia and Nok are about the same price. Is one better than the other?

No.  But one of them at least has a hideous owner.

If the price is the same I'd go with Nok

Does Nok have a smaller carry on size requirement?

 

Not that I'm aware of. I thought all the carriers within Thailand were the same for bag size. Think I've flown them all and never had one scrutinize my bag size/weight 

In my case Nok had free baggage allowance where Air Asia didn't. Found space between seats bigger on Nok vs Air Asia, but interestingly, the Nok seat belts some 10 cm shorter. Nok flies Boeing 737 on busy routes, while Air Asia uses Airbus 320. Air Asia is more punctual (according to FlightRadar). Air Asia also seems to have less dated aircraft, from the look of the equipment wear (leather on seats, etc) but both acceptable. I wouldn't mind booking either of them.

Given a choice between these two airlines, I would choose Nok Air. They just seem to treat passengers better.

I absolutely would not fly Nok. They often cancel flights as I've read they pay so little they are short of pilots. The last two times I used them, before I gave up, their flight from Khon Kaen to Bangkok was over an hour late and they used a propellor plane. And whenever I'm at DMK, using Air Asia, there are always, always cancelled Nok flights on the board.

Nok's absurd cockpit livery is sufficient to make me question the airline's general reputation...

Pricing and scheduling are the main considerations for me on short domestic flts. For a bit of fun you could always compare recent performance on Flight Radar .....ooh and sometimes Nok Air offers a complimentary cup cake !😁

Air Asia Thailand, runs the same as budget airlines in the UK, the difference between them is great. Both Ryan Air and Easy Jet fleece you for any infringement you take from Tickets to cabin bags and food etc.. They wouldn't last 5 minutes in Asia. Asian /Thai people would walk rather than put up with it. plus most Thais who use their service take the chance of a cheap hold bag when booking as they think about their needs and wish little confrontation. You will notice this when tourists travel with Air Asia thinking they can buck the system. The difference is they can obtain hold baggage with little extra expense. The fares are too high at present and the airline will lower prices when the government gives them some discounts on fuel taxes and landing fees, which they will do because they tend to listen to the people of Thailand. something that will never happen in the UK or Europe.

IFlew with vietjetair this morning. I use thrm a lot now Thaismile are no more and find them good. 

On this route both airlines have been fine for me over the years. Not had a cancellation but if there is, at least plenty of daily flights on a competitive route. On routes with only one or two flights a day, not so good if flight cancelled.

I ended up booking Air Asia as it had a flight from Suvarnabhumi.